1

THE KINETIC THEORY OF GASES

Boyle’s Law: Investigating the dependence of Volume on Pressure (Temperature

kept constant)

The diagram below shows the apparatus which gives a direct reading for both the volume
and pressure of a fixed mass of gas.

Air is trapped in a glass tube by a column of oil. The oil is supplied from a reservoir,
where it can be pressurized by using a tyre pump. The pressure above the oil in the
reservoir is read by using a Bourdon pressure gauge. The pressure above the oil in the
reservoir is transferred to the trapped air in the glass tube. The experiment is performed at
room temperature to keep the temperature constant.

Since no air is pumped into the reservoir, the gauge reads atmospheric pressure, (100
KPa). The pressure, P, is increased by pumping air and the volume, V, is recorded from
the vertical scale.

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The graphs below demonstrate the relationship between pressure, P, and Volume, V.

This relationship is stated as Boyle’s Law:

The volume of a fixed mass of gas is inversely proportional to its pressure if the
temperature is constant.

Boyle’s Equation:

P1 V1 = P2 V2

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Charles’ Law: Investigating the variation of the Volume of a gas with Temperature
(Pressure kept constant)

The diagram below shows the apparatus which gives a direct reading for both the volume
and temperature of a fixed mass of gas.

A column of air is trapped by a bead of concentrated sulfuric acid in a capillary tube. The
tube was then attached to a half-meter rule by using rubber bands.

This apparatus is placed inside of a beaker containing water and it is kept upright by
using a clamp-stand. The water surrounding the capillary tube is heating with a bunsen
burner, while stirring with the thermometer. The readings of the thermometer, T, and the
volume, V, were taken at 15oC intervals.

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The graphs below demonstrate the relationship between Volume, V, and Temperature, T.

This relationship is stated as Charles’ Law:

The volume of a fixed mass of gas is directly proportional to its absolute temperature
(on the Kelvin scale) if the pressure is constant.

Charles’ Equation:

V1 = V2
T1 T2

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Pressure Law: Investigating the variation of the Pressure of a gas with Temperature
(Volume kept constant)

The diagram below shows the apparatus which gives a direct reading for both the
pressure and temperature of a fixed mass of gas.

The apparatus consists of a large flask filled with dry air connected by pressure tubing to
a Bourdon pressure gauge. The bulb is placed inside a large container and is completely
filled with water. The pressure of the air is recorded over a wide range of temperatures
including 0oC and 100oC.

The graphs below demonstrate the relationship between Pressure, P, and Temperature, T

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This relationship is stated as Pressure Law:

The pressure of a fixed mass of gas is directly proportional to its absolute temperature
if its volume is constant.

Pressure Equation:

P1 = P2
T1 T2

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Gas Law: (For a fixed mass of gas)

Gas Law Equation:

𝑷𝟏 𝑽𝟏 𝑷𝟐 𝑽𝟐
=
𝑻𝟏 𝑻𝟐

Ideal Gas Law

No gas obeys the laws exactly. However, the laws provide a fairly accurate description of
how gases behave under different conditions of pressure, temperature and volume.

One useful concept is the Ideal (or perfect) Gas Law. The following equation can be used
for gases which exactly obey the gas laws:

𝑃𝑉 = 𝑛 𝑅𝑇

where P is pressure (Nm-2 or Pa)

V is volume (m3)
n is the number of moles of gas (mol)
R is the universal molar gas constant (8.314 JK-1mol-1)
T is the temperature (K)

(N. B. In order for a gas to obey this equation, it must be subject to the following
assumptions. There are no forces between the molecules of the gases and the internal
energy of the gas is entirely kinetic and only depends on its temperature.)

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Let N, be the number of particles in a substance. Then we can say that the number of
moles of the substance, n, is given by the equation:

𝑁
𝑛=
𝑁𝐴

where NA is Avogadro’s number (6.023×1023)

Then the Ideal Gas Law can be rewritten as:

𝑁
𝑃𝑉 = 𝑅𝑇
𝑁𝐴

But we also know that Boltzmann Constant (k) which is a gas constant per molecule
(1.38 ×10-23 JK-1); can be calculated by using:

𝑅
𝑘=
𝑁𝐴

Hence we can also state the Ideal Gas Law as:

𝑃𝑉 = 𝑁𝑘𝑇

Kinetic Theory Equations

Kinetic Theory is used to explain all the gas laws by observing the movement of the
particles. Several assumptions are made to derive these equations:

 The molecules of the gas can be assumed to be point molecules. This means that
they have negligible volume.

 There is no intermolecular forces of attraction. The particles are in random motion

and the average speed of the random motions produce a constant force. (constant
force → constant pressure)

 Collisions are (perfectly) elastic, the particles rebound after collision and kinetic
energy is conserved.

 Impact time is minimum. The rate of change of momentum during the impact
gives an average force provided that the time of impact is much less than the time
between impacts.

 All molecules of a particular gas are identical.

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 Newton’s Three Laws are adhered to.

 There is a sufficiently large number of molecules for statistics to be meaningfully

applied.

Pressure of a gas can be given be given by the following equations:

Equation 1:
1 ̅̅̅2
𝑃= 𝜌𝑐
3
where 𝜌 is the density of gas
̅̅̅
𝑐 2 is the mean squared speed of molecules of gas

Equation 2:
1
PV = 𝑀𝑔 ̅̅̅
𝑐2
3
where 𝑉 is the volume
𝑀𝑔 is the mass of gas

Equation 3:
1
PV = 𝑁𝑚 ̅̅̅
𝑐2
3

where 𝑁 is the number of molecules

𝑚 is the mass of a molecule

We can also state the mean (average) translational kinetic energy per monatomic
molecule of a gas by using

1 ̅̅̅2 3 𝑅
𝐾𝐸 = 𝑚𝑐 = 𝑇
2 2 𝑁𝐴
Hence
3
𝑚𝑒𝑎𝑛 (𝑎𝑣𝑒𝑟𝑎𝑔𝑒) 𝑡𝑟𝑎𝑛𝑠𝑙𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝐾𝐸 𝑜𝑓 𝑎 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒 = 𝑘𝑇
2
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𝟏 ̅̅̅𝟐 )
Deriving Kinetic Theories of Gas Equations (𝑷 = 𝝆𝒄
𝟑

Let us consider a closed circuit cubical container of length, L. each molecule of the gas
has a mass, m. Consider a single molecule which is moving towards Wall X and its x-
component of velocity is, u.
The molecule has a x – component of momentum, m1u1 towards the wall X. when the
molecule collides with Wall X, it changes the direction of momentum.
Since the collision is elastic, it will rebound at the same speed and the momentum will be
-m1u1. Hence the change of momentum due to collision with the wall is 2mu1.
The molecule has to travel a distance 2L (from Wall X to Wall X again) before it collides
with Wall X again.
We can calculate the time that the molecule takes by using the formula

𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝑡𝑖𝑚𝑒 =
𝑠𝑝𝑒𝑒𝑑

2𝐿
𝑡=
𝑢1

We can also calculate the force which is the rate of change of momentum:

𝑐ℎ𝑎𝑛𝑔𝑒 𝑜𝑓 𝑚𝑜𝑚𝑒𝑛𝑡𝑢𝑚
𝐹=
𝑡𝑖𝑚𝑒

2𝐿
𝐹 = 2𝑚𝑢1 ÷
𝑢1

𝑚𝑢1 2
∴ 𝐹=
𝐿
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Hence the pressure at X can be calculated by using the equation

𝐹𝑜𝑟𝑐𝑒 𝑎𝑡 𝑋
𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑎𝑡 𝑋 =
𝐴𝑟𝑒𝑎 𝑎𝑡 𝑋

𝑚𝑢1 2
𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑎𝑡 𝑋 = ÷ 𝐿2
𝐿

𝑚𝑢1 2
𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑎𝑡 𝑋 =
𝐿3

If there N molecules in the container and their x – components velocity are

𝑢1 2 , 𝑢2 2 , … , 𝑢𝑛 2

Then the total pressure on the Wall X is

𝑚(𝑢1 2 + 𝑢2 2 + 𝑢3 2 + ⋯ + 𝑢𝑛 2 )
𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑎𝑡 𝑋 =
𝐿3

Hence
𝑚𝑁𝑢̅̅̅2
𝑃=
𝐿3

where ̅̅̅
𝑢2 is the mean squared velocity in component x – direction
m is the mass of molecules
N is the number of molecules

𝑚𝑁
Since mN is the total mass of a gas in the container the total density is 𝜌 = 𝐿3

Therefore we can rewrite the equation of pressure at X as

𝑃 = 𝜌𝑢̅̅̅2

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If ̅̅̅
𝑐 2 is the resultant velocity of a molecule in x, y and z planes; then the components of
velocity can be represented by u, v and w respectively.
We can therefore state that
̅̅̅
𝑐 2 = ̅̅̅ ̅̅̅2 + ̅̅̅̅
𝑢2 + 𝑣 𝑤2

Where ̅̅̅
𝑐 2 is the mean square velocity of molecules
̅̅̅̅
𝑢 2 is the mean square velocity in x – direction

̅̅̅̅
𝑣 2 is the mean square velocity in y – direction

̅̅̅̅
𝑤 2 is the mean square velocity in z - direction

Since most of the molecules are moving randomly, we can state

̅̅̅̅
𝑢2 = ̅̅̅
𝑣 2 = ̅̅̅̅
𝑤2

Hence
̅̅̅ 1 2
𝑢2 = ̅̅̅
𝑐
3

Finally we can state that the pressure at X can be determined by using

1
𝑃= 𝜌𝑐̅̅̅2 [as required]
3

𝟏
Deriving Kinetic Theories of Gas Equations (𝑷𝑽 = 𝟑 𝑵𝒎 ̅̅̅
𝒄𝟐 )

Recall that to calculate Pressure at X by a molecule , we use the equation

𝑚𝑢1 2
𝑃𝑥 =
𝐿3

since V = L×W×H
𝑚𝑢1 2
𝑃𝑥 =
𝑉

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The pressure of many molecules moving in one plane, can be calculated by

𝑚𝑢1 2
𝑃1 =
𝑉

𝑚𝑢2 2
𝑃2 = ………
𝑉

𝑚𝑢𝑁 2
𝑃𝑛 =
𝑉

Hence the total pressure is given by 𝑃𝑇 = 𝑃1 + 𝑃2 + 𝑃3 + ⋯ + 𝑃𝑁

Therefore
𝑚(𝑢1 2 +𝑢2 2 +𝑢3 2 +⋯+ 𝑢𝑁 2 )
𝑃𝑇 =
𝑉

̅̅̅2 ) can be
We can say that the mean (or average) of N molecules velocity, denoted by (𝑢
calculated by

̅̅̅
2
(𝑢1 2 + 𝑢2 2 + 𝑢3 2 + ⋯ + 𝑢𝑁 2 )
𝑢 =
𝑁

Therefore
𝑁 ̅̅̅
𝑢2 = (𝑢1 2 + 𝑢2 2 + 𝑢3 2 + ⋯ + 𝑢𝑁 2 )

Hence
𝑚𝑁 𝑢̅2
𝑃𝑇 =
𝑉

Since the motion of the molecules in a gas is totally random, the pressure exerted in each
place is the same

𝑁𝑚𝑢̅̅̅̅̅
2 𝑁𝑚𝑢̅̅̅̅̅
𝑦
2 𝑁𝑚𝑢̅̅̅̅̅
2
𝑥 𝑧
𝑃𝑇 = = =
𝑉 𝑉 𝑉

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If we let ̅̅̅
𝑐 2 be the resultant velocity of all three components, then

̅̅̅
𝑐 2 = ̅̅̅̅̅
𝑢𝑥 2 + ̅̅̅̅̅
𝑢𝑦 2 + ̅̅̅̅̅
𝑢𝑧 2

And we can deduce that

̅̅̅ ̅̅̅̅̅
𝑐 2 = 3𝑢 2 ̅̅̅̅̅2 ̅̅̅̅̅2
𝑥 = 3𝑢𝑦 = 3𝑢𝑧

(because all the molecules travel at the same velocity in each plane)

Hence
1 ̅̅̅2 ̅̅̅̅̅2
𝑐 = 𝑢𝑥 = ̅̅̅̅̅
𝑢𝑦 2 = ̅̅̅̅̅
𝑢𝑧 2
3

̅̅̅̅̅
𝑁𝑚𝑢 𝑥
2
And we can rewrite the equation 𝑃𝑇 = as
𝑉

1 2
𝑁𝑚 ̅̅̅
𝑐
𝑃𝑇 = 3
𝑉

Therefore we can conclude that

1 2
𝑃𝑉 = 𝑁𝑚 ̅̅̅
𝑐 [as required]
3

Average Translational Kinetic Energy of Monatomic Molecules

The mean kinetic energy of a molecule of an ideal gas is given y

𝑡𝑜𝑡𝑎𝑙 𝑘𝑖𝑛𝑒𝑡𝑖𝑐 𝑒𝑛𝑒𝑟𝑔𝑦

𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝐾𝐸 =
𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠

We know that
1
𝑃𝑉 = 𝑛 𝑅𝑇 and 𝑃𝑉 = 𝑁𝑚 3 ̅̅̅
𝑐2

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We can say that

1
𝑁𝑚 𝑐̅̅̅2 = 𝑛 𝑅𝑇
3
𝑁
But we know that 𝑛 = . So we can rewrite the equation as
𝑁𝐴

1 𝑁
𝑁𝑚 ̅̅̅
𝑐2 = 𝑅 𝑇
3 𝑁𝐴

𝑅
Given that 𝑘 = the equation becomes
𝑁𝐴

1
𝑁𝑚 ̅̅̅
𝑐2 = 𝑁𝑘𝑇
3
1 ̅̅̅2
𝑚𝑐 = 𝑘𝑇
3

𝑚 ̅̅̅
𝑐2 = 3𝑘𝑇
1
Since 𝐾𝐸 = 𝑚𝑐̅̅̅2
2
1 ̅̅̅2 3
𝑚𝑐 = 𝑘𝑇
2 2

Hence
3
𝑚𝑒𝑎𝑛 (𝑎𝑣𝑒𝑟𝑎𝑔𝑒) 𝑡𝑟𝑎𝑛𝑠𝑙𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝐾𝐸 𝑜𝑓 𝑎 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒 = 𝑘𝑇
2

N.B. From using the equation derived above, we can make the following statements:

 The total kinetic energy in 1 mole of an ideal gas can be calculated by

3
𝐾𝐸𝑇 = 𝑅𝑇
2

 The total kinetic energy of n moles of an ideal gas can be calculated by

3
𝐾𝐸𝑇 = 𝑛𝑅𝑇
2

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